1. Field of the Invention
The present invention relates to a multiplex transmission system for data transmission between multiplex nodes which are connected to common signal transmission lines.
2. Description of the Related Art
Multiplex transmission systems of this type include LAN (local area network) transmission systems which combine CSMA/CD (carrier sense multiple access/collision detection) and NDA (non-destructive arbitration) or use tokens. As a typical LAN, a CAN (controller area network) or the like is used for data transmission in an automobile, for example.
One such multiplex transmission system is described in, e.g., Japanese Patent Application No. H2-113751 (Laid-open Publication No. H4-10828). This system is provided with terminal circuits which include terminal resistors. These resistors are connected to the respective both ends of multiplex transmission lines which constitute this system.
Due to the use of the terminal resistors, in a conventional multiplex transmission system, failure of one multiplex transmission line influences the other normal transmission lines, and the damaged transmission line cannot be separated. According to the system described above, therefore, a plurality of multiplex nodes, each including a transmission control circuit and a reception control circuit, are connected to one another by means of three common multiplex transmission lines. One of the multiplex nodes detects failure when the node can not communicate with other multiplex nodes, and a voltage generator circuit of this multiplex node changes the voltage of any one of the multiplex transmission lines, thereby changing the state of data transmission between the individual transmission lines. Based on this change of the transmission state, the transmission control circuit and the reception control circuit of each multiplex node can achieve multiplex transmission of data.
According to this multiplex transmission system, the reception control circuit must switch the multiplex transmission line for data reception in changing the transmission state, so that the configuration of the circuit is complicated, and there is such a risk that it takes much time to manufacture it. If the circuit configuration of this system is complicated, moreover, the ground potential difference between the multiplex nodes may be too small for accurate data transmission, in some eases.
According to this multiplex transmission system, furthermore, transmission waveforms collapse in the transmission control circuit, due to the difference in switching speed between drivers on the source current and sink current sides, so that the reception control circuit sometimes cannot normally receive the data. If one of the terminal resistors of this system is disengaged, moreover, reflected waves are produced on the multiplex transmission lines, so that the transmission waveforms collapse, possibly preventing normal data reception in the reception control circuit.